Bullous pemphigoid (BP) is an autoimmune subepidermal blistering disease characterized by autoantibodies and an inflammatory infiltrate at the lesional site. BP autoantibodies belong to IgG and IgE isotypes and recognize two hemidesmosomal proteins, BP180 and BP230. In vitro and in vivo studies have demonstrated that anti- BP180 autoantibodies fix complement and are pathogenic. NC16A, an extracellular domain of BP180, is the primary target of pathogenic autoantibodies. Eosinophils are usually prominent, and the dermal infiltrate also contains neutrophils, mast cells and lymphocytes. However, roles of these key innate immune players remain unknown. Lack of a usable in vivo system is a major obstacle for studies of innate immune responses in BP using patient-derived autoantibodies. Our lab currently developed a humanized BP180 mouse stain (termed NC16A mice), in which the mouse BP180NC14A domain is replaced by the human BP180NC16A domain. More significantly, neonatal NC16A mice injected with anti-BP180 autoantibodies develop skin disease that mimics key immunological features of BP. The objective of this proposal is to study the role of eosinophils and mast cells in BP using our newly developed NC16A mouse model. The overall goal of this project is to increase our understanding of the innate immunity of BP and how it relates to the functions of innate immune system players in inflammation and autoimmunity. In Aim 1, we will determine whether mast cells are required for BP. Aim 2 is to study the role of eosinophils and to determine functional interaction between eosinophils and mast cells in BP. In Aim 3, we will study the role of proteolytic enzymes of eosinophils and mast cells in BP blistering. Since this proposal integrates both disease mechanism studies and preclinical trials, the findings are expected to have a significant impact on the treatment of patients with BP.